The free fatty acid content of whole grain flour increases during storage, which subsequently results in rancid flavor and short shelf life of the whole grain flour and products made with the whole grain flour. Lipase is the enzyme in whole grain flour that hydrolyzes lipids to produce free fatty acids. Whole grain wheat flours containing bran and germ are less stable than white refined wheat flours because generally the enzymes responsible for causing production of free fatty acids and rancidity, and the lipids upon which the enzymes act are largely removed with the bran and germ during milling of the whole grains or berries to produce white refined wheat flours. Storage of whole grain wheat flours for as little as 30 days at 75° F. can result in the development of undesirable odors and flavors in products made with the whole grain flour. Concurrent with the development of off-flavors is an increase in the amount of free fatty acids in the flours, correlated with increased rate of oxygen uptake in the flours and the formation of the oxidative components of rancidity. Decreasing particle size increases the rate and extent of the deterioration of grain components. Heat and moisture treatment is commonly used to inactivate enzymes responsible for flour deterioration, although it is recently shown to contribute to oxidative rancidity as measured by hexanal formation, a common marker used to detect oxidative rancidity, in oat flour. This increase in oxidative rancidity is believed to be due to disintegration of cellular structures, such as by milling and heat and moisture treatment, that tend to stabilize lipids, or due to inactivation of heat-labile antioxidants. Also, the use of heat and moisture to inactivate enzymes tends to result in protein denaturization and starch gelatinization which can adversely affect protein functionality and starch functionality which in turn can adversely affect dough machinability and baking characteristics. Increasing stabilization temperatures, moisture contents and treatment times to achieve greater enzyme inactivation tends to exacerbate problems with protein functionality and starch functionality.
Lipase, which causes hydrolytic rancidity in milled products of sound, ungerminated wheat, is found almost exclusively in the bran component. The other key lipid-degrading enzyme, lipoxygenase (LPO), is present almost exclusively in the germ and also is involved in the development of rancidity. Thus, bran-containing wheat flours or graham flours are much more susceptible to the development of rancidity than are white flours which contain little or no bran and germ.
Enzyme-catalyzed lipid degradation that occurs in high extraction wheat flour, causing rancidity in such flour, is believed to occur by the action of lipase followed by the action of LPO. It is believed that when lipase, the enzyme found almost exclusively in the bran portion of the grain, is activated during milling, it reacts with unstable oils naturally occurring in the grain and breaks down the unstable oils to free fatty acids (FFA). This process may take weeks or even months. Then, LPO, the enzyme found almost exclusively in the germ portion of the grain, oxidizes FFA in the presence of oxygen, producing volatile breakdown products such as peroxides that, in turn, generate rancid aldehydes. In the absence of moisture, oxidation of FFA is also a very slow process and can take up to several weeks until noticeable amounts of rancid aldehydes can be detected. However, in the presence of moisture, or water, that is normally added to wheat flour in large amounts during the dough work-up stage, enzyme-catalyzed oxidation of free fatty acids tends to proceed to a great extent very quickly, causing formation of large amounts of rancid aldehydes in a matter of just a few minutes.
Accordingly, there is a long felt need for a method of producing a whole grain flour that is stabilized against enzymatic degradation.